The invention herein described was made in the course of a contract with the Office of Naval Research. This invention resulted from a study directed towards defining the maximum endurance achievable in a mobile, closed system, chemically fueled power plant. The stipulation of closed system required that the power plant exchanges no material whatsoever with its environment, thereby excluding for example air breathing systems or systems employing chemical reactions having gaseous end products which must be discharged to the environment. The mobility stipulation implies that the overall plant weight and volume will be the key determinants of endurance.
This invention is directed toward energy limited systems vice power limited systems, i.e., systems requiring enough energy to operate at full power for tens or hundreds of hours, rather than only a few hours or less. This means that the dominant concern is obtaining a low weight and volume of the total fuel and oxidant (reactants) which have to be carried, rather than a low weight and volume for the energy conversion devices.
The weight and volume of reactants necessary for a given amount of endurance are determined primarily by two factors: the energy released by the chemical reaction, both per unit weight of reactants and per unit volume of reactants; and the efficiency of converting this energy to the form of energy desired.
It is well established in the prior art that the reaction of most light metals with oxygen has a very high energy release per weight of reactants. It is also well known that electrochemical conversion-- such as occurs in a fuel cell achieves a high efficiency of converting reaction energy to electrical energy, since it is not subject to Carnot limitations. There have been numerous attempts to merge these two concepts to achieve an overall highly efficient, high energy density energy conversion system. A frequent technique has been to react one of the light metals (or their hydrides) with H.sub.2 O: the alkali metals will displace hydrogen from water, whereas the alkaline earth metals and certain others will displace hydrogen from steam. The hydrogen is then reacted in a hydrogen - oxygen fuel cell. However, the hydrogen evolved represents only roughly one half of the total reaction energy which is released by the metal-water reaction. The remainder is manifested as thermal energy.
The shortcoming of this approach heretofore has been that only the hydrogen has been further converted to useful energy; the thermal energy has either been rejected as waste heat or ignored. The reason for this is that the hydrogen generating reaction has been carried out at low temperatures, where Carnot limitations preclude converting a significant part of the thermal energy to a more useful form of energy. This low temperature has heretofore been required, since the metal or metal hydride fuel has been used in the solid state, and if the H.sub.2 O became hot enough to evaporate, it would escape with the product hydrogen.
This invention initiated with the simple objective of improving the prior art energy conversion systems in which hydrogen was generated from metallic fuels and burned in fuel cells. The improvement was to be accomplished by converting to useful form some of the thermal energy of the hydrogen producing reaction. In order to do this, it is necessary to generate the hydrogen at elevated temperatures, where most of the conventional metal fuels are in the liquid state, and to incorporate a heat transfer means in the hydrogen generator. The generator designed to meet these objectives has turned out to have much broader applicability than originally envisioned: it can be used to generate many gases other than hydrogen; and when used as a hydrogen generator, it makes possible several unusual energy conversion systems other than the straight forward combination of a fuel cell and closed cycle thermal engine, with each different energy conversion system having its own special advantages.